This invention relates to cobalt metal powder as a binder metal for the production of diamond and/or hard-metal tools and/or wear-resistant coatings and to composite sintered articles produced therefrom.
It is known that cobalt metal powder can be produced by atomization of the molten metal. Japanese patent application 53-093 165 describes the production and use of atomized cobalt metal. According to this document, a collected atomized crude product is treated by grinding and shock-tempering to obtain the desired hexagonal/cubic phase ratio. Grinding processes add to the cost of the cobalt metal powders, and are also a source of impurities.
Although cobalt metal powders can be produced extremely inexpensively by atomization from the melt, the powders obtained in this way are completely unsuitable as binder metals, for example for the production of diamond tools, because, they do not form dense composite sintered articles of sufficient hardness at typical sintering temperatures of 800.degree. to 900.degree. C. on account of the spheroidal particle shape and the particle size.
The unsatisfactory performance properties of hot-pressed composite sintered articles of atomized cobalt metal powder are mainly attributable to the inadequate compressibility of the prepressed blanks on account of the spheroidal particle shape, the relatively narrow particle size distribution and the coarse primary particles (FIG. 2). The necessary density of at least 8.5 g/cm.sup.3 is not obtained by hot pressing either.
By contrast, cobalt metal powders with an FSSS (i.e. Fisher Subsieve Sizing Method; see e.g., American Society of Testing Materials (ASTM) Procedure No. B330 for a typical description of FSSS measuring procedure) value of 3 to 5 .mu.m, so-called 400-mesh powders (FIG. 1 herein), suitable as a matrix material, can be obtained by reduction of oxygen-containing cobalt compounds with hydrogen at elevated temperature. These powders derive their name from the acceptance of the powder by a 400-mesh sieve. Powders such as these meet the requirements which the matrix metal for composite materials is expected to satisfy in terms of hardness and sinter density. However, 400-mesh powders have an extremely high percentage of impurities. It is generally known in this regard that aluminum, calcium, sodium, magnesium and silicon readily form stable oxides with the oxygen of the cobalt metal powder. These stable oxides can cause unwanted porosity in diamond segments.
In the case of hard metals, porosity-induced reductions in strength can occur if the impurities mentioned above and sulfur are present in excessive amounts. Accordingly, cobalt metal powders with low impurity contents are required for both applications. Depending on the amount of purification work carried out in the metallurgical preliminary stages, the purity of cobalt metal powders can be adapted to meet requirements. The expense involved in the production of particularly pure cobalt metal powders is of course considerable; powders such as these are therefore extremely expensive.
It is an object of the present invention to provide a cobalt metal powder which would not have any of the disadvantages of the powders described above.